This invention generally relates to the measurement of electrical properties and more specifically to apparatus which detects and displays electrical phase difference.
There are several phase detection and display devices known in the prior art. Some of these are adapted for use with instruments for measuring the frequency of musical notes. In one, a high frequency oscillator produces an output clock signal at a selected frequency. A series of frequency dividers and an octave selector switch provide a means for generating a reference signal at a selected subharmonic frequency. The tuning aid combines this reference signal and an audio signal representing the note being tuned either to generate an audible beat note or to deflect a pointer on an indicating meter. Unfortunately, these aids lose accuracy as the tuned note comes into frequency with the reference. When the beat rate decreases below 20 Hz, and especially 1 Hz, the audible beat note becomes inaudible. Similarly, an indicating meter uses a frequency-to-current converter so the current level goes to zero at a zero beat. As the current approaches zero, the visual indication becomes less accurate. Both types of display, therefore, lose accuracy at the very time it is most necessary.
In another unit, the tuner attaches a piezoelectric transducer to a particular string or a sounding board to produce a corresponding electrical signal that is applied to the vertical deflection plates of a cathode ray tube. A selector switch, crystal controlled oscillator and a series of frequency dividers generate a selected reference signal which energizes the horizontal deflection plates of the tube. In using this circuit, one apparently assumes, erroneously, that a piano generates a constant, repetitive wave form. In fact, a piano string generates an extremely complex wave form with a fundamental frequency and partials slightly out of tune with each other but often of the same magnitude. Furthermore, the component frequencies are not necessarily constant in relative magnitude because a string vibrates in many modes, each with its own damping constant. These factors cause the waveform to change continuously, so the display is difficult to interpret.
Another problem relates to dynamic response. Initially, the amplitude of the signal is sufficient to drive the display off the screen. As the tone dies out, the input to the vertical deflection plates falls below the minimum level necessary for generating a usable display. An obvious solution is installing a variable gain amplifier to maintain the output at a constant value. However, a circuit which provides satisfactory results over the wide range of conditions and waveforms which the piano generates is difficult to attain in practice. If the variable gain circuit actually tracks the decay, it may follow the waveform and provide a dc output signal. Therefore, this solution is not practicable especially in view of the non-linear parameters or conditions and the short interval for a readable display. This effective dynamic range further complicates tuning because adjusting a string while monitoring the display is very difficult.
Still another tuning aid receives the audio signal from a piano and generates a corresponding electrical signal to energize the blanking or Z axis circuitry of a cathode ray tube. A circular generator energizes X and Y axis deflection plates with a reference frequency so the electron beam describes a circle on the screen. If a note is in tune with the reference, the audio signal blanks and unblanks the electron beam during the same part of each revolution to thereby display one arcuate segment. A second harmonic input signal produces two such arcuate segments; a third harmonic input signal, three segments; and so forth. If a given note is not exactly harmonically related to the reference, the segments rotate. The direction of rotation indicates whether the note is sharp or flat while the speed of the rotation indicates the difference in frequencies. As notes in the upper piano produce a display with a number of segments, the spaces between adjacent sectors diminish; and the absolute frequency deviation which produces a persistent display tends to decrease. Furthermore, alternately blanking and unblanking the beam produces an indefinite segment termination on the screen. When the frequency deviation is small, the indefinite termination makes it difficult to determine whether the edges of the segments are moving. When notes in the lower range of the piano are tuned, the tuner must try to adjust while the tuning aid responds to harmonics, since subharmonics of the reference frequency generate complete circles on screen.
Therefore, it is an object of this invention to provide a circuit for measuring phase differences.
Another object of this invention is to provide a circuit for measuring and displaying phase differences.